It is common knowledge that the use of nickel-titanium instruments in root canal preparation has become widespread in recent years.
These instruments are used inside the root canal and operated with a continuous rotary movement induced by a “counter-angle” handpiece connected to a dental endo motor that enables a speed of rotation in the range of 250 to 350 revolutions per minute.
A continuous rotation at this speed enables great efficiency and excellent working rates to be achieved.
This speed of rotation can be used with Ni—Ti instruments, not with conventional steel instruments, because the Ni—Ti alloy has a characteristic superelasticity, and consequently assures the flexibility and elasticity (or shape memory) needed to enable the rotating instrument to advance through the root canal, circumferentially cutting the dentine without being deformed, even in the case of having to follow curved trajectories.
The continuous rotational movement of a spiral-shaped blade inside a canal gives rise to a spontaneous forward displacement of the instrument, in much the same way as a screw being turned inside a suitable medium.
This tendency of the instrument to advance as it rotates gives rise to the need for the dentist to “control” this forward displacement in order to avoid an excessively rapid, “uncontrolled” progression that would carry a risk of the instrument jamming or even breaking
This control is normally exerted by means of an “incremental” forward displacement, approximately 1 mm at a time, achieved manually by the dentist, i.e. the instrument is allowed to advance 1 mm and then withdrawn slightly before it is allowed to advance again, and so on.
Despite the use of this advancing technique, it nonetheless sometimes happens that the instrument suddenly accelerates out of control as it advances, engaging with an excessive length of root canal wall, and consequently becomes jammed and even risks breaking inside the root canal. This effect is known as “threading”.
Since each instrument has different, precise diameters starting from the tip (every instrument has a given tip diameter and conical taper, measured in hundredths of a millimeter) and each instrument is part of a series of different-sized instruments (in particular, of increasing sizes in the case in point), the depth at which this excessive engagement may occur—and the consequent risk of “threading”—can be calculated for each of the instruments.
A first solution to this “threading” problem was proposed by company FKG and adopted in an instrument marketed by the name of RaCe®. In this case, the full length of the active, cutting edge of the blade comprises alternate 2-3 mm lengths of spiral with a given first pitch and 2-3 mm lengths of spiral with a different pitch.
In addition, there are instruments currently available on the market that are characterized by two cutting blades, an incremental pitch, and a non-cutting tip.
In particular, the latest instruments have been made with the following sequence (the first figure refers to the diameter of the instrument and the second to its taper angle): 0.10 mm—4%; 0.15 mm—5%; 0.20 mm—6%; 0.25 mm—6%. The introduction of this innovative sequence has led to the production of Ni—Ti instruments that perform distinctly better than those of the previous state of the art.
The above-mentioned solution still fails, however, to prevent any occurrence of the previously-described technical problem of “threading”. A first solution to this problem has been provided by the same Applicant in the Italian patent application No. RM2009A000045 (extended as an International patent application PCT/IT/2010/00018), wherein the proposed solution involves manufacturing Ni—Ti instruments, preferably with a dual blade, with an incremental pitch and a non-cutting tip, and with a gap in the blade at a given distance from the tip designed to interrupt the advancing action of the blade and thereby prevent any “threading”.
It is also well known in endodontics that increasing attention has been paid in recent years to the preparation and cleansing of the last apical millimeters of a root canal.
The dimensions of the apical foramen are often around 20-25 hundredths of a millimeter, while the millimeters of canal adjacent thereto are wider in diameter: in a horizontal cross-section, the shape is almost always an oval with its greatest diameters often exceeding 30 hundredths of a millimeter at 1 mm from the tip.
Many root canal preparation methods relying on Ni—Ti instruments involve the use of instruments that are 0.20 or 0.25 mm in diameter at the tip and with a taper angle of 4% or 6%.
When one of these instruments reaches the apex of the canal, it often leaves areas untouched in the final millimeters of the canal because the diameter of the 0.25 mm—4% and 6% instruments 1 and 2 millimeters away from the apex is narrower than the larger diameter of the canal.
One solution could consist in further increasing the conical taper of the instruments, but this would make the instrument too large in the remaining portion, so that the instrument would consequently be excessively rigid and could become more difficult to control in its forward displacement, and scarcely conservative in the coronal portion of the canal.
Another solution is the one adopted for the Protapers, which have a conical taper that is more marked in the 3 mm length nearest the tip and more limited in the remaining portion of the instrument.